0:02 hi everyone welcome to the next video
0:04 discussing reactions today we're going
0:06 to be learning about reactions to a
0:08 geometry we're going to explain the
0:10 concept of Stoichiometry as it pertains
0:13 to chemical reactions and we're going to
0:15 learn how to use balanced chemical
0:17 equation to derive stoichiometric
0:19 factors which relate the amount of
0:22 reactants and products
0:24 so in previous videos we've learned
0:27 about balanced chemical equations and we
0:28 know that there's a great deal of
0:30 information that we can learn from them
0:32 for example our balanced chemical
0:34 equations show the chemical formulas
0:37 right of the reactants and products
0:40 involved in the reaction and we learned
0:42 how our chemical equation would also
0:44 include the correct phases for those
0:46 reactants and products
0:49 we similarly learned that when we have a
0:51 balanced chemical equation that the
0:53 coefficients the numbers written in
0:55 front of the reactants and products tell
0:57 us the relative numbers of these
0:59 chemical species and today we're going
1:01 to learn that using those coefficients
1:04 we can derive quantitative relationships
1:06 between the amounts of reactants and
1:09 products and that this is known as the
1:11 reactions stoichiometry
1:14 so when learning about Stoichiometry
1:15 it's helpful to think about something
1:18 familiar which is similar in concept and
1:21 that is food preparation or the use of
1:23 recipes so let's say that we're trying
1:26 to make pancakes and we take our box of
1:30 bisquicks and look at the recipe on the back
1:30 back
1:33 so the recipe might say that for one cup
1:36 of pancake mix and three quarter cup of
1:38 milk and one egg that we could make
1:40 eight pancakes and we can take that
1:43 information and write it in the form of
1:44 an equation
1:46 and this gives us the specific
1:49 proportions that are needed of our
1:52 reactants and product right our mix milk
1:56 and eggs to the product of Pancakes and
1:58 we know that that proportion is
2:00 necessary so that our pancakes aren't
2:03 too runny or aren't too thick so it
2:05 gives us the ideal amounts for our
2:07 specific product
2:10 now let's say that we have our equation
2:13 our recipe and instead of eight pancakes
2:16 we want to make two dozen pancakes we
2:17 know that we need to increase
2:21 proportionally the amount of foods in
2:24 our recipe right so we could think to
2:27 ourselves well since 24 which is two
2:32 dozen is equal to eight times three then
2:34 this tells me that I can take my recipe
2:37 that gives eight pancakes and triple it
2:41 right multiply all of my quantities by three
2:42 three
2:44 so this would tell me that instead of
2:47 one cup of mix then I would need three
2:50 cups right instead of three-fourths a
2:53 cup of milk I would need nine fourths
2:56 and instead of one egg I would need
2:59 three eggs and that would give me 24
3:02 pancakes right so that's one way that we
3:05 might think of tripling our recipe in
3:08 this case or we could write it
3:11 quantitatively by realizing that what we
3:20 and I can use the proportions in my
3:24 equation to learn that when I generate
3:34 and so writing it this way the pancakes
3:37 cancel and what I'm left with would be
3:41 24 divided by eight or three
3:45 and my units here would be eggs right so
3:47 tripling our recipe is easy if we know
3:48 that we have
3:51 you know just triple the pancakes that
3:52 we're trying to find but if we're
3:55 looking at a different non-integer value
3:57 which we might see in our chemical
4:00 equations doing the calculation on the
4:02 bottom would be would be a better approach
4:04 approach
4:06 so now let's bring this around to our
4:09 chemical equations so we can think of
4:12 them again in a similar way to recipes
4:14 so from our balanced chemical equation
4:17 we can determine the amount of reactant
4:20 needed to yield a given amount of
4:23 product so in our recipe we knew how
4:25 many eggs we would need in order to
4:28 generate a certain amount of Pancakes in
4:29 a similar way with this chemical
4:32 equation we know for example how much
4:35 hydrogen gas we would need to react in
4:37 order to generate a certain amount of
4:40 ammonia gas NH3
4:43 we can also learn the amount of one
4:46 reactant required to react with a given
4:48 amount of another so for example if I
4:52 have a certain amount of hydrogen gas I
4:55 can calculate how much nitrogen gas I
4:58 would need to react with it in order to
5:00 use up all of the hydrogen gas for
5:02 example right if I have a certain amount
5:04 of mix I can figure out how much milk
5:07 and eggs I can add to it in order to use
5:08 it up completely
5:11 so the coefficients in our balanced
5:13 equations are going to be used to derive
5:16 what we'll call stoichiometric factors
5:18 that are going to allow us to do these
5:22 calculations to determine to determine
5:23 these things so let's look at our
5:25 chemical equation that has been on the
5:29 slide this tells us that we need one
5:32 mole of nitrogen gas to react with three
5:35 moles of hydrogen gas to yield two moles
5:38 of ammonia gas and that's one way for us
5:40 to think of it in terms of moles we
5:41 could also think of it in terms of
5:45 molecules right one molecule of nitrogen
5:48 gas can react with three molecules of
5:51 hydrogen gas to yield two molecules of
5:54 ammonia gas so it really just depends on
5:56 the unit that we're thinking in we just
5:59 always have to keep this two to three
6:02 ratio for example between the ammonia
6:04 and the hydrogen gas
6:07 so using this ratio we're able to write
6:11 stoichiometric factors that will relate
6:13 two different species in our chemical
6:17 equation for example we could write that
6:20 we would have two ammonia molecules for
6:22 every three hydrogen molecules that
6:24 react we could also think of this in
6:27 terms of dozens right two dozen ammonia
6:30 molecules for every three dozen hydrogen
6:32 molecules that react you can see that
6:34 the unit really doesn't matter as long
6:36 as we have the same unit in both the
6:39 numerator and the denominator and then
6:41 finally and this is probably the
6:43 stoichiometric factor that you would use
6:46 the most right when we write it in terms
6:48 of moles so here we would write two
6:51 moles of ammonia are generated for every
6:54 three moles of hydrogen gas that are
6:57 reacted I want to also point out that
6:59 these factors don't always have to have
7:02 right for example the product in the
7:04 numerator or the hydrogen gas in the
7:06 denominator how you write it right which
7:09 species you put in the numerator and
7:10 which you put in the denominator really
7:13 depend on the calculation that you're
7:16 trying to do and which species you're
7:18 sort of canceling out and which one
7:20 you're trying to calculate so that will
7:23 depend on your exact equation
7:25 so next we'll learn how to use the
7:27 stoichiometric factors to perform
